The present invention comprises a linebar and saw assembly for sawing logs or cants having varying amounts of sweep from piece to piece. It is especially useful for increasing the recovery of saleable lumber from sweepy cants or logs.
Within a few more years, the last of the old growth timber available for harvesting in the United States will have been cut. Most of the sawmills which formerly depended on large old growth logs have already had to convert the equipment in their mills to that which is suitable for handling much smaller logs at high throughput volumes. One common characteristic of second growth timber is that the logs usually have varying amounts of sweep or curvature. The presence of sweep results in yields which are significantly lower than those which are attainable from straight logs. This is because much of the curved portion must be machined off in order to produce cants or lumber having the usual configuration of a rectangular parallelepiped.
Most small log sawmills orient the log prior to the primary breakdown saw so that the greatest curvature is either up or down (horns up or horns down) rather than side to side. These opening cuts may be either made by saws or by chippers which reduce the sides of the log to wood chips suitable for pulping. The result is a cant having parallel faces on two sides. As noted earlier, cants sawn in this fashion tend to have appreciable sweep. Sweep is here defined to mean the curvature on the concave edge of the cant when the cant is placed on one of its flat faces. While occasional instances of extreme sweep occur, in most cases sweep is rarely greater than about 100 mm in a cant approximately 5 m long (about 4 in. in 16 ft.). Most typically, it will be about half of this amount.
Man has had to attempt to cope with geometric irregularities in logs ever since he began to utilize trees. Thus, it is not unexpected that the prior art shows previous attempts to deal with sweepy cants and to devise schemes for improving the yield from cants of this type.
As general background to the present invention, reference can be made to U.S. Pat. Nos. 259,661 to Bowker and 1,263,443 to Lien. Both of these inventors devised schemes for sawing wooden barrel hoops along a path that was precisely parallel to the outside surface of the log. This was deemed necessary in order to reduce the presence of cross grain which would reduce the strength of the hoop. Somewhat more pertinent is Great Britain Pat. No. 545 of 1852. In this sawing device, a tree was placed on a carriage which could be moved in a circular arc with respect to a saw. The purpose here appears to be to cut complex curved and/or angled ships timbers. However, it appears inherent in the invention that cuts could be made parallel to the surface of a curved log.
Japanese Pat. No. 49-7557 shows a device used for "sawing around the curve" of a cant which contains sweep. The cant is first sawn to produce one good edge parallel to the original surface. After this point, additional boards can be taken off parallel to this surface by manually steering the cant as it emerges from the saw.
Swedish Pat. No. 33,098 is a sawing device having feed rolls which can be canted so that the axes lie at an angle and intersect at a point corresponding to the arc of a curve along which a cut is to be made. U.S. Pat. No. b 3,685,556 to VanSickle takes a somewhat different approach. This inventor shows a device for use with a shop band saw for cutting predetermined contoured pieces for straight stock to make; e.g., Christmas tree stands. More conventional approaches are described in U.S. Pat. No. 3,665,984 to Ackerfeldt, U.S. Pat. No. 4,416,312 to Ostberg and Swedish Pat. No. 306,415. These, in general, deal with positioners for optimizing yields from curved cants by straight line sawing. The patent Ackerfeldt is valuable for the background it gives to this type of approach.
Whle it has been noted that increased yields can be obtained by "sawing around the curve", until very recently nothing has been done to make this a practical approach in a modern, high-speed sawmill. The closest examples can be found in U.S. Pat. Nos. 4,144,782 and 4,219,056 to Lindstrom and U.S. Pat. No. 4,373,563 to Kenyon. The earlier of the two Lindstrom patents is of particular interest. In the device described, a log is fed into what is preferably a single band saw along a path which follows the curvature of the log. The log lies on a roll case with two pairs of opposing vertical guide rollers upstream from the saw. These guide rollers embrace the cant as it is fed into the saw. They are mounted on frames transversely displaceable to the path of the log so that the log is directed along a curve to which the saw blade is tangent. Lindstrom further notes that the bowed boards resulting from around-the-curve sawing will generally flatten out when they are kiln dried. Kenyon shows a somewhat similar system. However, the cant is oriented by lateral pressure applied only to one side of the cant by a roller located upstream from the point of sawing.
To the present inventor's knowledge, the devices taught by Lindstrom and Kenyon have had limited commercial application although their use has not become widespread. This may possibly be because they are of limited use in a high-speed sawmill which may typically run 10-20 cants per minute through a gang saw.
In two earlier applications commonly assigned with the present one and in which the present applicant is a co-inventor, the applicants have successfully dealt with the problem of maximizing yield from cants having significant amounts of sweep. One method utilizes apparatus disclosed in Ser. No. 793,925, filed Nov. 1, 1985, which is hereby incorporated by reference. This application shows a linebar which can be bent by the sawyer to varying degrees of curvature which will correspond to the curvature of the particular cant being sawn. At a point opposite the saw arbor a tangent to the linebar always lies parallel to the plane of the saws. A second application, Ser. No. 814,137, filed Dec. 27, 1985, which is also herein incorporated by reference, discloses the use of a linebar having a fixed curvature. This curvature is equivalent to the average sweep found in a large sample of cants being processed by the sawmill. As is the case with the other linebar, a tangent to the linebar at a point opposite the projected axis of the saw arbor lies parallel to the plane of the saws.
Both of the above linebars have had a positive effect upon recovery in mills where they can be incorporated. Unfortunately, sometimes the first of these devices can not be used due to physical limitations found within sawmills. A typical small log sawmill might have a gang saw linebar 7.3 m (24 ft) in length. For a cant having about 50 mm sweep in 4.9 m length (2 in in 16 ft) the infeed end of the linebar must be deflected about 460 mm (18 in) from the tangent line. Space is not always available in a sawmill to accommodate a linebar deflection of this magnitude. The second device described above gives a major improvement over straight line sawing but cannot take full advantage of the possible yield increase since it is configured for a cant of "average" sweep.
The present invention is designed to overcome the problems of space limitations which would prevent one of the above linebars from being utilized. In addition, it has other advantages which will now be described.